As a consequence of the European directives, the purification techniques currently employed are aiming to limit discharges into the natural environment of untreated wastewater, stations having to treat all collected water apart from that resulting from exceptional downpours. The common characteristic of these treatment stations is that of carrying out purification by activated sludge, comprising a treatment basin in which oxygen is transferred to the "water/biomass" mixture. This transfer is necessary for the development of the purifying biomass.
Furthermore, increased reliability is demanded by the European directives in question and therefore requires purification stations to be strictly monitored and to be maintained in the best possible state of operation, with a maximum available purifying capacity (that is to say with a biomass "in the top of its form" and with equipment in a perfect state of maintenance and operation).
One of the major causes of malfunction of purification stations is the deficiency or insufficiency of the oxygen supply; this oxygenation deficit is manifested long term by the appearance of anaerobiosis phenomena, resulting in the development of filamentary bacteria in the biomass and then the appearance of foams, the phenomenon of performance degradation being all the more rapid as it is autoaccelerated.
The efficiency of the aeration system is usually measured, before the purification station is commissioned, using in vitro measurement techniques which absolutely do not take into account the influence of the biomass on the oxygen transfer. This is because the transfer coefficient (which in this case we will call the standard transfer coefficient) is usually measured in clarified water and in the presence of sodium bisulphite--a reducing agent for oxygen; it is thus possible to control gas-liquid transfer, but to the exclusion of any biological phenomenon. On the other hand, when the biomass is "installed" in the aeration basin, the said biomass operates as a biological oxygen pump and it is then possible to obtain a corrected transfer coefficient .alpha.k.sub.L a, where .alpha. represents a correction coefficient having a value of between 0.5 and 0.9, which corrected transfer coefficient takes into account the biological operation, the differences in viscosity between the mixed liquor and the clarified water, and the differences in hydrodynamics resulting from operation in the presence of biomass. Once the station is running, tricky measurement techniques have to be used to determine the amount of oxygen transferred to the water/biomass mixture, while still obtaining only instantaneous values, giving, for example, no information about a possible drift in the efficiency of the aeration system.
The systems according to the prior art which are usually employed for monitoring and/or controlling the operation of wastewater purification stations are of various types: